Compositions and devices for thermo optically controlled switching and filtering

ABSTRACT

The invention relates to polymer compositions which enable thermooptic control of signal attenuation in the ultraviolet, visible and near infrared (NIR) regions of the electromagnetic spectrum, and devices incorporating such compositions. The compositions are derived from polymer mixtures which exhibit a cloud point phase transition at a temperature in the range of a thermooptically controlled device such as a programmable waveguide attenuator, a programmable neutral density filter, or an optically absorbent switch. An especially preferred embodiment of the invention comprises a mixture of a high molecular weight chlorotrifluoroethylene fluid and a wax with an “ON-state” insertion loss of below 0.1 dB/cm and an extinction ratio of 22 dB/cm in the 1550 nm NIR telecommunication band.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to the field of thermooptically activematerials used in optical components and technology. More particularly,the present invention relates to the fields of optical waveguides,optical filters, optical switches, laser optics, neutral densityfilters, optical attenuators, flat panel displays, and projectiondisplay devices.

A persistent difficulty in the field of optical polymer materials is thehigh attenuation of such materials when they are in their inactive, ormaximally transmissive state, sometimes denoted the “ON-state”. Forexample, many polymer plastics have been developed for use in passiveoptical waveguides for telecommunication signals in the near infrared(NIR), particularly at 1300 nm and 1550 nm. Even though such polymersare intended to be optimally transmissive at all times (i.e. always intheir ON-state), attenuation levels of up to 0.5 dB/cm are typical forconventional “low loss” materials. The absorption in such polymers isusually due to the presence of carbon-hydrogen bonds that exhibitstrongly absorbing anharmonic vibrational resonance overtone bands inthe NIR. The halogenated analogs of carbon-hydrogen polymers, whereinthe hydrogen is substituted by a higher mass halogen such as fluorine,bromine, iodine, or chlorine, exhibit much weaker NIR absorption and aretherefore more highly transmissive. Halogenated polymers have beentested as passive waveguide media that exhibit improved attenuationlevels of under 0.1 dB/cm. In an embodiment of the present invention,preferred for applications in the NIR, the composition compriseshalogenated polymers in order to realize low attenuation.

So-called “active” polymers are used in optics in order to manipulatecharacteristics of optical signals by means of external control inputs,which affect the optical properties of the polymers. The polymercompositions are suitable for use in thermooptically active opticalcomponents because of their large thermooptic coefficient for the indexof refraction, dn/dT, or rate of change of refractive index withtemperature. In an example of an electro optically active polymercomposition for use in an optical component, radio frequency (rf)electrical control inputs, rather than thermal inputs, are used to allowhigh-speed phase modulation of optical signals propagating within thepolymer. An important figure of merit for any such actively controlledpolymer medium is that it have minimal total absorption when switched tothe ON-state. Ideally, it will have the best achievable ON-stateabsorption performance for the passive polymer media mentioned above, ora figure of merit, usually denoted in the art as “insertion loss”, ofless than 0.1 dB/cm.

The expected absorption for the prior art electrooptically controlledchromophoric polymers is at least 10 dB for a 1 cm active polymer lengthor an insertion loss of 10 dB/cm. These materials, when used in aMach-Zehnder interferometer configuration can provide a maximumabsorption in the OFF-state of 30 dB. The difference in these twoattenuation levels is a second important figure of merit for anyswitched medium, denoted the “extinction ratio”. In this case, thedifference between the maximum OFF-state absorption and the minimumON-state absorption yields an extinction ratio of 20 dB. Since theirdevice has an active polymer length of 1 cm, their extinction ratio perunit length is 20 dB/cm. Extinction ratio performance for the presentinvention was measured to be 22 dB/cm, a level equal to or improved overprior art media.

Other optical switches described in the prior art are comprised ofthermooptically controlled polymer materials that exhibit insertion lossof 0.6 dB/cm and are limited to an extinction ratio of 20 dB total. Inthe prior art, U.S. Pat. No. 6,208,798 discloses a polymer cladding thatis thermooptically controlled to permit leakage and hence attenuation ofoptical signals from the waveguide core; a preferred embodiment isdescribed wherein the insertion loss is 3 dB for a design which providesan extinction ratio of 20 dB. The materials of the present invention,when configured as the core of a simple thermooptically controlledwaveguide, rather than the more complex configuration of a Y-branchwaveguide, a Mach-Zehnder interferometer, or a thermoopticallycontrolled leaky waveguide cladding, would provide an insertion loss ofbelow 0.1 dB/cm and 22 dB of extinction ratio for every centimeter ofwaveguide length. Thus, the present invention promises a significantreduction in complexity, and a substantial improvement in performancefor thermooptically controlled devices over that of the prior art.

A further advantage of the present invention is that it exhibits a highthermooptic control coefficient. That is, for a modest change in thecontrol parameter (temperature), a large change in signal attenuation isrealized. The measured thermooptic control coefficient for compositionsof the present invention is in the range of 6 dB/cm/° C. When combinedwith the low insertion loss of the present invention of less than 0.1dB/cm, this figure of merit implies that for a relatively small changein temperature along a waveguide, a relatively large change inattenuation can be achieved. For example, a ten-centimeter longwaveguide attenuator using a composition of the present invention wouldexhibit an insertion loss of 1 dB and 60 dB/° C. thermoopticcoefficient. So, with a control temperature change of only 0.2° C., sucha device could switch 10 dB in attenuation. Allowing such a small changein temperature is a significant advantage over the prior art because itpermits, for a given thermooptic heating/cooling power per unit length,a more rapid switching speed in any device utilizing compositions of theinvention.

Yet another advantage of the present invention is that it enables designof components that have attenuation that is smoothly varying and variesonly gradually across its wavelengths of operation. Some thermoopticallycontrolled devices must be optimized for performance in narrow bands ofspectrum because their attenuation performance relies on resonance ornulling phenomena. For example, devices which rely on the thermoopticchange of refractive index of a polymer waveguide arm of a Mach-Zehnderinterferometer must be optimized for a center wavelength. Variation ofoperating wavelength by more than one quarter of the center wavelengthvalue renders the device unable to achieve its ideal phase cancellationpoint. In the prior art disclosed in U.S. Pat. No. 6,165,389, aswitching medium is described which depends on a volume phase transitionphenomenon in the thermooptically active material; a preferredembodiment exhibits variation in optical absorption which is stronglyresonant in wavelength. By contrast to the foregoing prior art examples,the present invention relies on bulk absorption due to a “cloud point”phase transition phenomenon; in addition, its absorption characteristicsvary only slowly across the entire wavelength band from 300 nm to 2000nm without any rapidly varying resonance attenuation features. Usingcompositions of the present invention, devices can be configured whichexhibit only small changes in optical absorption across wavelength,which we denote “insertion loss flatness”. A preferred embodiment of theinvention, for example, achieves a worst case insertion loss flatness ofless than 0.01 dB/nm in the vicinity of NIR telecommunication bands overthe wavelength range of 1200 nm to 1600 nm.

A further characteristic of some optical polymer media such as liquidcrystal polymer media is that they exhibit birefringence, meaning thattheir index of refraction is significantly different for the twoorthogonal signal polarizations. This in turn leads to a deleteriousfeature known as polarization dependent loss (PDL). In some applicationssuch as polarization independent waveguide switches, or free spaceneutral density filters, it would be desirable to realize behavior for aswitch medium that is non-birefringent in nature and exhibits minimalPDL. The cloud point mechanism which gives the present invention itsswitching characteristics is intrinsically non-birefringent in natureand may therefore be advantageous in such applications.

The composition of the present invention may be used to fill an opticalwaveguide structure. Such structures are commonly designed to ensurepropagation in a single waveguide mode. There are structures used in thetelecommunications field, which are also based on propagation ofmultiple modes, known as multimode waveguides. The compositions of thepresent invention may be used to fill both single mode and multimodewaveguide devices. The prior art of optical waveguides as described inU.S. Pat. No. 5,692,088, also includes optical waveguides comprising aliquid core. Compositions of the present invention may exist in aliquid, a quasi-liquid or a solid state over a portion of theiroperating temperature service range. The distinctive feature of thecompositions of the present invention is that they exhibit a transitionin optical absorption, which may be controlled through variation intemperature. Devices that will benefit from the incorporation of theinvented composition into waveguide structures include waveguideswitches, programmable waveguide attenuators, and controllable waveguideshutters.

In some applications for the present invention, the electromagnetic wavepropagates in an unguided free-space mode rather than in a guidedwaveguide mode. Projection display optics, high power laser optics, andother optical systems sometimes require neutral density filters, orcontrollable attenuators, or shutters which control the free-spaceamplitude of an optical wavefront. In the prior art,reflective/absorptive free-space elements have been fabricated usingtransparent liquid crystal arrays, or using microelectromechanicalswitch (MEMS) elements, or using electrically controlled crystalelements such as Pockels cells. In one embodiment of the presentinvention, a free-space attenuating screen is formed by filling the gapbetween two closely spaced transparent sheets of glass with the inventedcomposition. The temperature of the composition is varied to permitswitching of the state of the filter through a range of neutral densityvalues. This approach may be carried further to allow, across thetwo-dimensional face of the filter, independent control of localizedattenuation. In this way, the present invention could be used to realizecontrollable beam shavers, image pixel amplitude equalizers, holographicimage attenuation controllers, and the like.

The prior art also contains many compositions in the art of manufactureof greases, gels, plastics, paints, and other compounds wherein themechanical properties of the compositions are altered by the addition ofpowder thickening agents such as nanopowder silica, oxides, soaps,clays, or other materials. In some embodiments of the present invention,it may be advantageous to increase the viscosity or stiffness of theinvented compositions by means of addition of these components. It ispreferred in these cases to select thickening agents that do not undulyincrease the level of haze in the composition. This may be accomplished,as is well known in the art of transmissive optical polymercompositions, by minimizing the effective particle size of thethickening agent, minimizing the volume fraction of the agent within thecomposition, and selecting agents with maximal optical clarity and withindex of refraction close to that of the base composition. In likemanner, fluid thickening agents such as tackifiers, or other highviscosity fluids which are miscible with the base composition and whichare well known in the art of lubricant formulation may be compoundedinto the base composition in order to increase its viscosity and reduceits tendency to migrate or flow once it is dispensed into the opticalassembly.

The prior art of compounding of greases, plastics, lubricants, and othercompounds also contains knowledge regarding the addition of smallpercentage quantities of additives or fortifiers, such materials beingincluded in compositions at up to 10% by weight. Additives may be eithersoluble or insoluble in the base fluid of the composition. Additivesserve to imbue the composition with a variety of improvements inproperties. An illustrative but incomplete list of types of additives isas follows: anti-oxidants, lubricity additives, surfactants, fluorescentdyes, absorbing dyes, electrical conductivity additives, and metalscavengers or passivators. Many such additives are available forpurchase from the additive chemical supplier base. Fortifiers caninclude insoluble species such as oxide powders, fluorescing powders,nanotubes, nanospheres, microspheres, pigments, liquid crystals, andother materials. It is envisioned that some such additives andfortifiers could be included in compositions of the present inventionprovided that the key optical performance parameters for the compositionsuch as insertion loss and extinction ratio are not unduly degraded. Itis further envisioned that compositions of the present invention couldbe employed as a bulk medium to physically support arrangements ofvarious fortifier species for the purpose of forming a photonic crystal,a Bragg reflector, a frequency doubling film, or numerous otherphotonically active materials.

The present invention permits the thermooptic control of signalamplitude in a waveguide or free space propagation configuration withsubstantial improvements in: ON-state insertion loss per unit length,extinction ratio per unit length, thermooptic control coefficient perunit length, insertion loss flatness, and polarization dependent loss.

The invention relies on the optical characteristics of a “cloud point”phase transition phenomenon. A “cloud point” is the property of a fluidor fluid mixture to change from clear to cloudy as it is cooled andpasses through a precipitation transition. The most familiar example ofthis phenomenon is the transition of paraffin wax from a clear fluid inits melted state to a hazy soft solid in its solid state. In the fieldof petroleum fluid technology, the cloud point is a temperature at whicha mixture of two or more fluids transitions from clear to hazy as one ofthe species precipitates out of solution as a microcrystalline wax.Since polymers generally comprise mixtures of a number of molecularweight species of related chemical structure, the cloud point transitionis often not sharply defined with respect to temperature. However, bycontrolling the range of molecular weight within narrow limits, thetransition can be sharpened, affording more dramatic thermooptic controland higher and more advantageous thermooptic control coefficient. As isknown in the art of design of synthetic waxes, it is also possible tovary the nominal molecular weight of a particular type of wax such thatits cloud point, and therefore the composition's nominal ON/OFFtransition temperature point, can be selected to be most advantageousfor a particular application. Molecular weight homogeneity of a polymerfluid or wax may be improved by techniques well known in the art ofpolymer manufacture such as distillation, wiped-film vacuumfractionation, supercritical fluid extraction, and the like.

It is unproven whether the thermooptic response of the present inventedcompositions are limited solely by the rate at which the controltemperature may be varied within the composition as limited by the heattransfer design of the device employing the composition. However, it isexpected that embellishments of the invented composition will serve tosharpen the thermooptic response and enable faster switching speeds.First, the range of the molecular weights of the wax portion of thecomposition may be controlled within narrow limits as described above.It is speculated but not proven that the addition of a small quantity ofinert solid fortifier powder to the composition will provide nucleationsites for condensation of the microcrystalline wax phase of thecomposition, improving the speed at which the composition can transitionfrom the ON to the OFF state. It is expected but not proven thatexternally applied pressure changes will also increase the thermoopticcoefficient of the composition. It is envisioned that such pressurechanges could be applied, for example, by passive means in a devicedesign by arranging for the structure which encloses the inventedcomposition to expand or contract the enclosed volume at a predeterminedrate with temperature. This could be accomplished, for example, bychoice of enclosing structure materials with coefficients of thermalexpansion and/or controlled heating and cooling which achieve thevolumetric changes needed to apply the desired static pressures.

In some applications of the present invention, it is advantageous toformulate a composition which is a high viscosity flowable fluid. Thisallows the material to be more easily poured or dispersed into smallspaces in the optical assembly. In the preferred embodiment of theinvention, a wax is dissolved in a flowable fluid to make a plasticizedmixture which is highly viscous but pourable under pressure. It isopaque at room temperature and clears to be highly transmissive attemperatures above 34° C. It may be desirable that compositions of thepresent invention remain highly viscous throughout the OFF portion oftheir service temperature range. This feature serves to immobilize theprecipitated wax species and prevent it from separating out as distinctphases through buoyant or agglomerative forces, thereby causing unwantedoptical performance effects. On the other hand, it may also be desirablein some applications to ensure that at the extreme lower limit of itsservice temperature range the material retain sufficient pliability toavoid formation of fractures within the composition or delamination ofthe composition from surrounding substrates. It is postulated, but asyet untested, that this requirement may be equivalent to requiring thatthe composition contain a non-freezing fluid component which serves toplasticize the wax at cold temperatures and prevent fractures ordelamination. The prefered embodiment of the invention contains such aplasticizing fluid component.

It is also possible to conceive embodiments of the optical devices ofthe present invention comprising solely a wax which is opaque andimmobile or semi-solid at room temperature without flowability, but isflowable above its cloud point.

If the wax and fluid are chosen to be of types that have viscosity whichdecreases rapidly with increasing temperature, the composition may haveadditional advantages for manufacturing. By heating such a fluid wellbeyond its operating temperature, the fluid may be made low in viscosityso that it may be more easily injected into the optical device; uponbeing cooled to its service temperature range the fluid mixture revertsto its higher viscosity and immobile state. It is a feature of thepreferred embodiment of the present invention that it has a highviscosity temperature coefficient in order to obtain thismanufacturability advantage.

It is envisioned that in some applications, such as outdoortelecommunications applications where the service temperatureenvironment reaches 85° C. or higher, that the desired cloud pointtransition would be set to be somewhat higher than this maximum servicetemperature, for example, at 90° C. This would allow the device designerto perform the thermooptic control by means of a heating element only,without the need for an active cooling element. As an example, apractical control approach would be to design the device package tooperate at a continuous internal temperature of 85° C., representing thestandby OFF-state. A stepped up heating state would be designed to boostthe temperature to a 95° C. to switch the composition to the ON-state.To switch from ON to OFF, the stepped up heating state would bedisabled, allowing the device to cool, via passive heat-sinking toambient temperature and return the composition to the OFF-state. Inother applications, where nominal ambient temperature is roomtemperature, a suitable cloud point transition would be chosen for thecomposition to be slightly above room temperature. As with the examplejust cited, one could use a control heater to raise the temperature tothe ON-state, and rely on equilibration with ambient temperature toswitch the composition to the OFF-state. It is a feature of the inventedcomposition that within classes of appropriate polymer fluids and waxes,that a wide range of cloud point temperatures can be selected to suit aparticular application. For example, with the preferred embodiment ofthe invention, higher and lower melting point waxes could be chosen tomix with the base clear fluid so as to move the cloud point higher orlower in temperature. Such design flexibility would be helpful, forexample, if the switch state while unpowered at ambient temperature werespecified to be fully OFF (sometimes called “normally open”), or fullyON (sometimes called “normally closed”). One application which wouldbenefit from a “normally open” version of the invented composition wouldbe a waveguide safety shutter for long haul telecommunications fibercarrying eye-damaging levels of infrared laser radiation.

The compositions of the present invention are comprised of at least onematerial component which has the following properties:

1) at the OFF temperature the material component (the “wax”) is anoptically opaque semi-solid or solid material (the “wax”);

2) at the ON temperature the wax is optically clear.

A preferred embodiment of the invented compositions are those which arecomprised of at least two miscible but distinct materials which have thefollowing properties:

1) at the OFF temperature one is an optically clear fluid (the “fluid”)and one is a wax;

2) at the ON temperature both materials are optically clear and they aremiscible in proportions of at least 1:100 (fluid:wax) but morepreferably are infinitely miscible;

3) at the OFF temperature the mixture of materials has a viscosity whichis sufficiently high to prevent separation of the phases of the twomaterials; preferably the OFF viscosity of the mixture is at least 1000cP, and more preferably is at least 20,000 cP.

A preferred embodiment of the invented compositions is one which isoptically transmissive in the near infrared in the ON state comprising:

1) ten to ninety parts by weight of a fully halogenated, substantiallybonded-hydrogen free, chlorotrifluoroethylene polymer fluid;

2) ninety to ten parts by weight of a fully halogenated, substantiallybonded-hydrogen free, high molecular weight chlorotrifluoroethylene wax.

Other combinations of materials can be conceived which follow theinvented prescription. For example, a thermoplastic composition whichexhibits microcrystallinity below its microcrystalline transitiontemperature will exhibit behavior vs. temperature which mimicscompositions of the present invention. In this sense, any material whichexhibits a microcrystalline precipitation vs. falling temperatureexhibits the properties of the “wax” component of the presentinvention's compositions, and can be used with optical devices of thepresent invention provided that such transitions are reversible andrepeatable. Examples of such embodiments of the present invention wouldbe optical devices incorporating the following: polytetrafluoroethylenecompositions, polyalphaolefin compositions, paraffin wax compositions,hydrocarbon microcrystalline wax compositions, polypropylenecompositions, partially or fully halogenated wax compositions, siloxaneresins or fluids, polyphenylether or polyphenylthioether resins orfluids, and the like.

For the purposes of the present invention, a thermooptically active orthermooptically controllable composition is a composition which can becontrolled to have an opacity which varies over a range of temperaturein a well-defined, reversible, and repeatable manner between an“ON-state” which is substantially transparent and an “OFF-state” whichis substantially opaque.

Unless otherwise specified, “optically clear” is taken to mean having anoptical transmission of greater than 90% over a 1 mm path length at theintended “ON state” temperature and at the intended wavelength oftransmission as measured using a grating spectrophotometer with aninstantaneous spectral bandwidth of 10 nm or less. Conversely,“optically opaque” is taken to mean having an optical transmission whichis less than 90% at the intended “OFF state” temperature and at thewavelength of intended use.

Unless otherwise specified, “ultraviolet” is taken to mean a range ofwavelengths from 100 nm to 400 nm, “visible” is taken to mean a range ofwavelengths from 400 nm to 750 nm, and “near infrared” is taken to meana range of wavelengths from 750 nm to 2000 nm.

Unless otherwise specified, “index of refraction” or “refractive index”is taken to mean the value of refractive index of a medium measured at25±0.5 degrees centigrade and 589 nm, as measured by the method of Abbérefractometry according to ASTM D-1218, or equivalent method. Thisdefinition is the convention in the optics industry, even when theintended wavelength of use of the material differs from 589 nm as is thecase with wavelengths in the near infrared.

Unless otherwise specified, a statement that two adjacent opticalmaterials are “index-matched” is taken to mean that they differ inrefractive index by no more than 0.06. By this definition, a fusedsilica fiber (refractive index=1.46) is index-matched to a compositionof the preferred embodiment of the present invention (refractiveindex=1.42).

Unless otherwise specified, “service temperature” indicates temperaturelimits or ranges at which or over which the optical performance criteriaare applied. The composition may also be subjected to temperatures aboveor below its service temperature limits, but without necessarily meetingits optical performance criteria.

Other objects, features and advantages of the invention shall becomeapparent as the description thereof proceeds when considered inconnection with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which illustrate the best mode presently contemplatedfor carrying out the present invention:

FIG. 1 is a schematic depiction of a waveguide attenuator containing athermooptically active composition of the present invention;

FIG. 2 is a schematic depiction of a neutral density filter containing athermooptically active composition of the present invention;

FIG. 3 is a graph comparing optical absorption vs. wavelength for thepreferred embodiment composition at various temperatures; and

FIG. 4 is a graph comparing optical absorption vs. temperature for thepreferred embodiment composition at 1550 nm.

DETAILED DESCRIPTION OF THE INVENTION

In many optically based systems, there is a need to actively control theopacity of a light-transmitting composition in order to switch off orsubstantially attenuate the signal. Prior art means for accomplishingthis function suffer from excessive insertion loss in the ON state,inadequate extinction ratio in the OFF state, inadequate thermoopticcoefficient, or inadequate insertion loss flatness vs. wavelength. Thepresent invention is intended to improve these figures of merit for athermooptically controlled composition and for devices incorporatingsuch compositions. Examples of fields which would benefit from theinvention are the field of telecommunication waveguide switches andwaveguide attenuators, and the field of programmable neutral densityfilters for free-space optics.

An illustration of the present invention in the field oftelecommunication waveguide attenuators is shown in FIG. 1. FIG. 1depicts the geometry of a programmable waveguide attenuator for thetelecommunications bands at 1300 nm and 1550 nm. The waveguide could beof the single mode or multimode type and could be straight as shown inFIG. 1, or curved. In cross-section, the waveguide could be circular,rectangular, semi-circular, or other arbitrary geometric shape as longas it supports the propagation of at least one guided electromagneticwave mode. For example, the waveguide could be formed within a planarstructure so as to have an approximately rectangular cross-section as iswell known in the art of planar integrated optics. Optical fibers (1, 3)or functionally equivalent planar optical waveguide runs are used toinject a signal at the input port (4) and retrieve the signal at theoutput port (5). The signal passes through the thermooptically activeregion (2) which extends from the input fiber to active region inputjunction (10) to the active region to output junction (11). A heatingelement (6) in close proximity to the active region (2) is designed toheat the composition (7) which fills the active region (2) from itsOFF-state to its ON-state. Heater control circuitry (not shown) ofarbitrary design is used to energize the heating element (6). Suchcircuitry may simply operate in a bi-static mode giving either full ONperformance, or full OFF performance. Or, the circuitry may be designedto vary continuously over fine temperature increments so as to preciselycontrol the opacity of the composition (7), allowing the device tooperate as a programmable precision attenuator. Alternatively, many suchheating elements (6), individually controlled, may be placed along thelength of the active region (2). In some embodiments it may be desirableto fabricate the structure of FIG. 1 within a substrate medium (8). Ifthe substrate medium (8), chosen for convenience of available substratematerials, has a refractive index which is higher than that of thecomposition (7), then improved waveguide definition and signalconfinement, especially at waveguide bends, will be achieved if theactive region (2) is clad with an optional thin coating of material (9)with refractive index less than that of the composition (7). Many suchcoating materials, made from both organic polymers and inorganicmaterials, are available from the optical materials industry. It is alsodesirable in some applications that the refractive index of thecomposition (7) be index-matched to the refractive index of the fibers(1,3) in order to reduce Fresnel reflection at the interfaces (10, 11).However, as is well known in the art of fiber splicing and in the art ofmicrowave and millimeter waveguide structures, the reflections can bereduced by designing interfaces (10, 11) which are modestly angled asshown in FIG. 1, stepped down in successive impedance matching steps, oreven tapered over a length of many fiber diameters (not shown in FIG.1). Another technique for overcoming imperfect index matching betweenthe fibers (1,3) and the thermooptically active region (7), is to inserta quarter-wavelength index matching transformer in the waveguide at theinterfaces (10, 11) as is well known in the art of microwave windowmatching transformers, and in the art of anti-reflection coatings forlenses. Alternatively, the composition in the active region (7) can bedesigned to be index-matched to the fibers (1,3). It will also berecognized to those skilled in the art of polymer waveguide design thatcompositions of the present invention, in particular the preferredembodiment composition, even if permanently operated in an ON-state,will serve as a pliable alternative to other low loss but rigidthermoplastics.

An illustration of the use of the present invention in the field ofneutral density filters is shown in FIG. 2. A free-space wave isincident from the left following the input ray path (12). It encountersthe outer face (13) of the neutral density filter assembly formed by asandwich of two planar sheets (14,15) of optical quality glass orplastic enclosing a region (16) of thermooptically active composition ofthe present invention. When the region (16) is thermooptically switchedto its ON-state, the incident ray passes through the region (16) andthen passes through the second sheet of glass (15) and continues in thedirection indicated for the output ray path (17). When the region (16)is switched to its OFF-state, the amplitude of the output ray (17) issubstantially decreased. If the region (16) is caused to be at atemperature which makes it partially transmissive, it will take on anintermediate neutral density state and cause a corresponding welldefined attenuation of the amplitude of the output light ray (17).Various mechanisms can be envisioned for controlling the temperature ofthe region (16) so as to adjust its opacity. These include: raising orlowering the ambient temperature of the entire device which contains theassembly of FIG. 2, placing a transparent resistive heating grid orcoating coplanar with and bonded to and across one or more of thesandwich surfaces (13,18,19,20), heating the entire sandwich assemblywith an externally directed infrared lamp or laser, or heating the outeredges of the sandwich (which lie outside the zone of interaction of thefree-space lightwave) by means of a heating or cooling lines or otherelements.

Various embellishments of the invention of FIG. 2 will be obvious tothose skilled in the art of optics, lens design and anti-reflectioncoatings. For example, the tilt angle (21) of the surface (13) withrespect to the incident light ray can be adjusted to be at anadvantageous angle. One such angle is that which allows the incidentlightwave to encounter the interfaces (13,18,19,20) at or nearBrewster's angle, minimizing reflections for one polarization. Inanother embellishment, one or more of the interfaces (13,18,19,20) aremodified to have anti-reflection coatings which serve to reduce Fresnelreflections between zones of non-identical refractive index.Alternatively, instead of providing anti-reflection coatings at theinterfaces (18,19), the composition used in the active region (16) canbe designed to be index-matched to the optical glass or plasticmaterials (14,15) which form the sandwich structure. In anotherembellishment the planar sheets (14,15) could individually be replacedwith more complexly-shaped structures such as curved lenses. In yetanother embellishment, the localized heating elements described abovefor heating the active medium (16) could be independently controlled toafford a control matrix of attenuation elements across the entire faceof the device. Such a configuration could find use as a display imageamplitude equalizer, or a holographic image amplitude equalizer. It willbe further obvious to those skilled in the art of imaging technologythat the invented device can be combined with other devices such as flatpanel displays, planar imaging detector arrays and the like so as toimpart advantageous properties to those other devices.

EXAMPLE Thermooptic Composition for Near Infrared Optical Components

A composition is prepared by heating the following ingredients to above40° C.: 40% by weight of Halocarbon HC-1000N, and 60% by weight ofHalocarbon Wax 600. The materials listed are available from HalocarbonCorporation, P.O. Box 661, River Edge, N.J. 07661 USA. The ingredientsare mixed after heating with a stir motor or other equipment suitablefor mixing high viscosity fluids. The mixture is allowed to cool to roomtemperature. If air or dust particles become entrained in the mixture,the composition may be reheated, filtered and deaerated using methodsfamiliar to those in the art of ultrafiltered aerospace greasemanufacture.

At room temperature, the invented compound is milky white in appearanceand has a soft putty-like texture. At room temperature it does not flowor migrate at rest but can be forced to flow under pressure through asyringe or other dispensing device. When heated to well above itstransition temperature, the viscosity of the composition decreasesrapidly, reaching a value below 500 cP at 100° C. which is convenientfor rapid dispensing into small devices in a manufacturing environment.

The thermooptic behavior of this formulation is described in FIGS. 3 and4. The optical absorption of the composition was measured using aVarian-Cary 500 grating spectrophotometer with a temperature controlledsample cuvette stage. At the OFF-state temperature of 25° C., theoptical absorption was measured to be in excess of 22 dB/cm at 1550 nm.At the ON-state temperature of 34° C., the optical absorptionrepresenting the ON-state insertion loss was measured to be less than0.1 dB/cm at 1550 nm, giving an extinction ratio of 22 dB/cm. Thethermooptic coefficient was measured to be 6 dB/cm/° C. at 1550 nm asshown in FIG. 3. The insertion loss flatness is described in FIG. 4across an operating band from 1200 nm to 1600 nm. The data show theabsence of resonances or nulls, indicating that a single devicecomprising the composition, such as those described in FIG. 1 or 2,could operate across the entire wavelength range. In the 1200 nm to 1600nm band, the worst case insertion loss flatness was measured to be lessthan 0.01 dB/nm.

While there is shown and described herein certain specific structureembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described except insofar as indicated by the scope of theappended claims.

What is claimed:
 1. An optical neutral density filter device comprising:an active medium, said active medium having a microcrystalline wax phasetransition from non-crystalline to semi- or fully microcrystalline at atemperature within the service temperature range of the neutral densityfilter device; a light path through said active medium; a housing forcontaining said active medium; and a heating device adjacent to saidhousing, said heating device providing a measured amount of heat tocontrol the transition of said active medium between saidnon-crystalline state, wherein said active medium prevents lighttransfer along said light path, and said semi- or fully microcrystallinestate, wherein said active medium allows light transmission along saidlight path.
 2. The optical neutral density filter device of claim 1,wherein said active medium has a microcrystalline structure that issubstantially free of hydrogen-bonded species.
 3. The optical neutraldensity filter device of claim 2, wherein said active medium is selectedfrom the group consisting of chlorotrifluoroethylene wax andbromotrifluoroethylene wax.
 4. The optical neutral density filter deviceof claim 1, wherein said active medium has a microcrystalline structurethat is partially or fully halogenated wax compositions.
 5. The opticalneutral density filter device of claim 4, wherein said active medium isselected from the group consisting of polyalphaolefin compositions,paraffin wax compositions, hydrocarbon microcrystalline waxcompositions, polypropylene compositions, siloxane resins, siloxanefluids, polyphenylether resins, polyphenylether fluids,polyphenylthioether resins and polyphenylthioether fluids.
 6. Theoptical neutral density filter device of claim 1, wherein the activemedium is held permanently within a range of temperatures which put itin a state of arbitrary fixed attenuation.
 7. The optical neutraldensity filter device of claim 1, wherein the active medium is heldpermanently within a range of temperatures which put it in a state ofarbitrary fixed attenuation wherein said range of attenuation furtherincludes a state which is substantially non-attenuating.
 8. Thewaveguide attenuation device of claim 1 wherein said active mediumincludes performance additives up to 5% by weight selected from thegroup consisting of: anti-oxidant, absorbing dye, fluorescing dye, metalscavenger, metal passivator, acid scavenger and mixtures thereof.
 9. Thewaveguide attenuation device of claim 8 wherein said active mediumincludes particle thickening agents up to 50% by weight selected fromthe group consisting of: silicon dioxide powders, oxide powders, liquidcrystals, fluorescent powders, microspheres, nanotubes, clays, metalpowders, conductive polymers, chromophoric polymers, ceramic powders andmixtures thereof.
 10. The waveguide attenuation device of claim 1wherein said active medium includes particle thickening agents up to 50%by weight selected from the group consisting of: silicon dioxidepowders, oxide powders, liquid crystals, fluorescent powders,microspheres, nanotubes, clays, metal powders, conductive polymers,chromophoric polymers, ceramic powders and mixtures thereof.